Cavity resonator for klystron tube



March 22, 1960 El (5..Jl\hlE5 CAVITY RESONATOR FOR KLYSTRON TUBE 2Sheets-Sheet 1 Filed Feb. 7, 1958 MUN IWIh. g

BEPIKAM GAE 145s RVVENTTHI BY iw, army I Arm/EV March 22, 1960 B. G.JAMES 2,929,955

CAVITY RESONATOR FOR KLYSTRON TUBE Filed Feb. 7, 1958 2 Sheets-Sheet 276 Jf a 5 f7 III 15A i f :5/% f3 a J 36 3 n A 4 W A3 J COLLECTOR.BfPTEA/M GLJA/MES INVENTOR.

:EII3 5 BY CAVITY RESONATOR FOR KLYSTRON TUBE Bertram Gordon James,Redwood City, Calif assignor to Eitel-McQullough, inc, San Bruno, Qalid,a comoration of California Application February 7, 1958, Serial No.713,913

9 Claims. (Cl. 315-=5.46)

This invention relates generally to beam tubes such as klystrons whichemploy cavity resonators and more particularly to an improved cavityresonator structure for such tubes.

In one type of klystron tube the cavity resonators associated with thevarious gaps are made up of two parts: a first part which is internal tothe evacuated envelope, and a second part which is external to theevacuated envelope and which carries the tuning means. A cylindricaldielectric window forms a part of the evacuated envelope and providesmeans for coupling energy from the internal portion of the cavity to theexternal portion. A tuning means within the cavity determines theresonant frequency thereof.

The configuration of the internal portion of prior ,art cavities is suchthat electric fields having relatively high components extendingradially outward from the gap are set up at the gap. These radial fieldcomponents tend to cause the secondary electrons produced by .theimpingement of primary electrons on some inner surface of the cavity tobe directed toward and bombard the dielectric window. The bombardment ofthe window causes heating of the window which often results in thecracking thereof and the consequent loss of the vacuum Within the tube.

Furthermore, the electric field configuration within the cavity is suchthat high voltage gradients exist at the window. This causes furtherheating of the window due to the high RF. losses in the window and thusincreases the danger of the cracking of the window, resulting indestruction of the vacuum within the tube. In addition, the energy lossin the window reduces the Q of the cavity.

It is an object of the present invention to provide a klystron having animproved cavity resonator structure.

It is another object of the present invention to provide a klystronhaving a resonant cavity which includes portions external and internalto the evacuated envelope of the klystron with a dielectric windowcoupling the two portions, in which the electric fields in the cavityare such that the heating of the dielectric window is reduced.

A more specific object of the invention is to provide a klystron havinga resonant cavity including internal and external portions coupled by adielectric window, in which electron bombardment of the window isreduced and the voltage gradient across the length of the window isreduced.

Referring to the drawing:

Figure 1 is .an elevational view of a four cavity klystron tube;

Figure 2 is an enlarged view, partly in section, of the portion 22 ofFigure 1;

Figure 3 is a sectional view schematically showing a cavity resonator inaccordance with the prior art;

Figure 4 is a sectional view taken along the lines 4-4 of Figure 2; and

Figure 5 is a sectional view showing another embodiment of the novelcavity resonator.

2,s29,955 Patented Mar. 22, 1960 with cavity resonators in accordancewith the subject invention is illustrated. The tube comprises agenerally cylindrical envelope having an electron gun 11 at one end anda water-cooled collector 12 at the other end. The electron beam isprojected from the electron gun to the collector and passes through adrift tube made up of a plurality of metallic sections 13, 14, i5, i6and 17 which extend axially of the tube and are spaced from each otherto form four gaps of which 18 and 19 are shown in Figure 2. Each ofthese gaps is bridged by a resonant cavity structure. Thus, fourcavities 21, 22, 23 and 24 are shown. A suitable cooling jacket 26surrounds the drift tube portions between the various tuning sections.Similarly, a cooling jacket 27 surrounds the collector 12. Water orother suitable cooling fluid may be introduced into the collector jacket27 at the inlet and outlet connections 28 and 29. Suitable inlet andoutlet connections (not shown) are provided for the jacket 26.

The resonant cavities are made up of two parts: an inner part which iswithin the evacuated envelope, and an outer part which is external tothe evacuated envelope and carries the tuning means. Dielectric windows31, one of which is shown, form a portion of the evacuated envelope andprovide means for coupling energy from the internal portion 32 of theresonant cavity to the external portion 33. The windows surround theassociated gaps and have their ends suitably sealed to the adjacent wallportions of the resonant cavity. Seals of this type are known in the artand will not be described in detail. Tuning means 34 are carried in theexternal portion of the resonant cavity as previously described.

The above described elements of the tube function in a manner well knownfor klystron tubes. Namely, an electron beam from the gun 11 isaccelerated by positive potential on its anode (not shown) and passesthrough the drift tube past the interaction spaces provided by the gaps,and finally terminates on the collector 12. The resonant cavities 2124serve as the frequency determining elements of the device. As previouslydescribed, the tuning cavities receive movable tuning plungers whichserve to tune the cavity over a particular frequency range. The tubeillustrated may function as an amplifier with the input signal formodulating the electron beam fed into the resonant cavity 21 and theamplified radio frequency output taken from the resonator 24 inaccordance with the usual practice in four-cavity type klystrons.

The tuning means 34 are moved by the tuning screws 36 which arethreada-bly received by the end plate 37. The other ends of the tuningscrews 36 are rotatably secured to the tuning means 34. The tuning means34 may be any suitable type of tuning plunger. It may, for example,carry contact members 39 which engage the adjacent walls of the cavity(Figures 2 and 4). One of the walls 43 includes adjustable inductiveiris opening 45 which serves to couple energy from the cavity resonatorportion into an associated waveguide secured to the flanges 46.Adjustable diaphragms 47 are driven towards and away from one another bythe lead worms 48 secured to ends of the shaft 49 to control the irisopening 45.

The tuning doors 34 are of open box-like construction and include endportions 50 having suitable configuration whereby they do not interferewith the opening 45 as they are moved.

A novel cavity resonator in accordance with the invention is shown indetail in Figure 2. The external portion 33 of the cavity includes apair of parallel walls 41 and 42. The internal portion 32 includes wallshaving inner surfaces 51 and 52 which are spaced from and facing eachother. Each of the surfaces defines the surasst-inns face of a truncatedcone with its axis coincident with the axis of the associated drifttube. The apex of the cone extends to a portion of the cavity adjacentthe gap 19 and the base of the cone extends to a portion of the cavityadjacent the ceramic window 31.

The advantage of a cavity according to this invention can best beunderstood by comparison with a cavity according to the prior art.

Referring to Figure 3, a prior art cavity 53 is shown associated withdrift tube sections forming a gap 54. The external portion of the cavity55 includes parallel wall portions similar to wall portions 41 and 42 ofFigure 2 and which receive a tuning means (not shown) similar to thatshown in Figure 2. The internal portion 56 of the cavity 53 is separatedfrom the external portion 55 by means of a ceramic cylinder 69. Theinternal portion 55 of the cavity 53 comprises parallel wall portions 57and 58 which are each sealed. to a drift tube section. The ceramiccylinder 60 serves as a window, coupling RF. energy from the internalportion 56 to the external portion 55 of the cavity 53. Thus, a cavity53 according to the prior art comprises a pair of parallel walls throughwhich drift tube sections project toward each other to form a gap 54.This structure results in a concentration of the electric field in thecavity 53 at the gap 54 due to p the close spacing of the drift tubesection's. Such concentration of the electric field at the gap 54 isdesirable since, it provides for increased interaction between theelectric field in the cavity and the electron beam of the klystron assuch beam traverses the gap 54.

However, if the electric field within the cavity is plotted it will befound to have the distribution represented generally by the dotted linesin Figure 2. Due to the concentration of the electric field at the gap54 the field adja cent the gap will have a strong radial component, asindicated by the curvature of the dotted lines adjacent such gaps 54.Due to spacecharge efiects, primary electrons in the beam will tend tospread and enter the cavity rather than proceeding on down the drifttube. Someprimary electrons at the edge of the beam will actuallybombard the ends of the drift tube sections adjacent the gap 54. Suchbombardment of the ends of the drift tube sections will result in therelease of large numbers of secondary electrons. Due to the radialcomponent of the electric field within the cavity, the secondaryelectrons produced by the bombardment of the drift tube will beaccelerated toward the ceramic window 60. The bombardment of the ceramicwindow 69 by the electrons, as mentioned above, will result in localizedheating thereof and possible cracking of the ceramic window due to thetemperature gradient established thereacross.

In order to decrease the effects described above it was customary inprior art cavities to provide the ends of the drift tube sectionsadjacent the gap with knife edges as for example by tapering the ends ofthe drift tube as shown at 59. According to the theory of the prior art,the purpose of such knife edges was to minimize electron bombardment ofthe ends of the drift tube sections and thus reduce the number ofsecondaries produced. However, the radial components of the electricfield adjacent the gap remained substantially unchanged.

Referring again to Figure 2 and particularly to the dotted linesrepresenting the electric field distribution in the cavity there shown,the advantages believed to be obtamed by the cavity according to thesubject inventionare readily apparent. it will be seen that the radialcomponent of the electric field adjacent the "gaps 19 (as represented bythe curvature of the dotted lines) is much smaller than in the prior artcavity 53. Thus, due to the decreased radial components in the electricfield, the acceleration of secondary electrons toward the ceramic window31 is decreased and thus the bombardment and consequently the heating ofthe ceramic window is reduced.

Furthermore, if we compare a cavity according to the subject inventionwith one according to the prior art (such as 53), each being designed tocover the same frequency range, additional advantages will be observed.With respect to the design of a cavity according to the subjectinvention to cover a given frequency range, it should be pointed outthat although the inclusion of the truncated cone surfaces 51 and 52 inthe internal portion 32 of the cavity represents a substantial decreasein volume compared to straight walls such as 57 and 58 in the prior art,the resonant frequency of internal portion 32 is not necessarilyincreased as might be expected. This is due to the fact that theinductance introduced into the portion 32 by the surfaces 51 and 52 isless than that introduced into the portion 56 of the cavity 53 by thesurfaces formed by the drift tube and end walls 57' and 58 due to thefact that the electrical path is shorter along the surfaces 51 and 52,whereas the capacity introduced into the portion 32 by the opposedsurfaces 51 and 52 is greaterthan that introduced into the portion 56 bythe end walls 57 and 58 due to the closer average spacing of surfaces 51and 52. Thus, the decrease in inductance in the portion 32 of the cavityaccording to the subject invention tends to be compensated by theincrease in capacitance.

The above described design features of a cavity according to the subjectinvention enables the use of a longer ceramic window while maintainingthe same or an increased tuning range. In other words, by employingtruncated conical surfaces 51 and 52 it is possible to use a longerceramic cylinder 31 without appreciably affecting the tuning range ofthe cavity or even with an increase in the tuning range. The use of alonger ceramic window is desirable since the voltage across the windowwill be distributed over a greater length thus decreasing the voltagegradient and therefore the danger of arcing. In addition, the amount ofheating per unit volume of the ceramic will be decreased in accordancewith the increased volume of ceramic, thus decreasing the possibility ofthe ceramic cracking due to heating effects. Increases in ceramic lengthof as much as 50% have beenrobtained according to the subject inventionwith no appreciable change in frequency range. Thus, the voltagegradient and heating of the ceramic may be correspondingly reducedwithout appreciable change in the tuning range of a cavity byconstructing such cavity in accordance with the subject invention.

Referring to Figure 5, another resonant cavity is shown. The cavityincludes an external portion 33 and an internal portion 32. The cavityshown includes walls 61 forming a part of the external cavity which maybe suitably clamped onto the klystron tube rather than being formedintegral therewith. The conical wall portions are formed by conicalsheet metal members 62 having their apexes suitably secured to theadjacent portions of the drift tube 63 and their bases suitably securedto the walls 64. A ceramic window 31 is suitably sealed to the adjacentwall portions in a manner well known to the art, and provides couplingbetween the internal and external portions of the cavity, as previouslydescribed.

Thus, it is seen that an improved cavity resonator is provided forklystron tubes having internal and external cavity portions with adielectric coupling therebetween. The configuration of the cavityresonator permits the use of longer cylindrical dielectric windows. Afield configuration in which the RF. losses in the windows are reducedand electron bombardment of the window is minimized is also provided.Further, the configuration provides a tuning cavity having an increasedQ and improved tuning characteristics.

What is claimed is:

1. An electron tube having an evacuated envelope, said electron tubecomprising a drift tube including a plurality of sections, spaced fromeach other to form at least one gap, an electron gun at one end of saidtube serving to project a beam of electrons through said drift tube,

a collector electrode at the other end of said drift tube and serving toreceive said electron beam, a cavity rescs v nator surrounding said gap,adielectric window sealed within said cavity resonator and forming apart of said evacuated envelope, said dielectric window dividing saidcavity resonator into two portions and serving to electrically couplesaid portions, the portion of said cavity resonator external to saidevacuated envelope including a pair of spaced walls having internalsurfaces parallel to each other and perpendicular to the axis of saiddrift tube, the portion of said resonator internal of said envelopehaving surfaces which define a pair of truncated cones one on each sideof said gap, said truncated cones being coaxial with said drift tube andhaving their bases adjacent opposite ends of said dielectric window andtheir apexes extending towards one another, said pair of truncated conesbeing coaxial with each other and with said,

drift tube, the spacing between said pair of walls being less than thedistance between the bases of said truncated cones.

2. A cavity resonator comprising an evacuated portion and a portionwhich is not evacuated, a dielectric cylinder separating said portionsfrom each other serving as a vacuum tight envelope member therebetweenand coupling electrical energy from one portion to the other, theportion of said cavity internal of said dielectric cylinder comprising apair of truncated cones with their bases adjacent opposite ends of saiddielectric cylinder and their apexes extending toward each other, saidpair of truncated cones and said dielectric cylinder being coaxial, thebases of such cones having a diameter approximately equal to theinternal diameter of the dielectric cylinder, and the external portionof such cavity comprising a pair of spaced walls, one adjacent each endof said dielectric cylinder, the length of said dielectric cylinderbeing greater than the spacing between said pair of walls.

3. A cavity resonator comprising a first portion and a second portion,said first portion being contained within a dielectric cylinder andcomprising a pair of truncated cones coaxial with said dielectriccylinder, the apexes of said truncated cones extending toward eachother, the bases of said cones being adjacent opposite ends of saiddielectric cylinder and having a diameter approximately equal to theinternal diameter of said dielectric cylinder, the second portion ofsaid cavity comprising a pair of spaced parallel walls external of saiddielectric cylinder and one wall adjacent each end of said dielectriccylinder, the length of said dielectric cylinder being greater than thespacing between said pair of walls.

4. A beam tube comprising an electron gun serving to project a beam ofelectrons, an electrode spaced from said gun, an RF. interaction meansinterposed between said gun and said collector and having an axis alongwhich said beam passes through said interaction means, said interactionmeans including an interaction gap, a cavity resonator surrounding saidgap, said cavity resonator having a first portion and a second portionseparated by a dielectric window, said dielectric window serving toelectrically couple the first and second portions of the cavityresonator, said second portion of the cavity resonator including spacedwalls having spaced parallel internal surfaces, and said first portionincluding internal surfaces which define a pair of truncated cones withtheir apexes extending towards one another and their bases extending toadjacent opposite ends of the dielectric window and having axescoincident with said axis along which said beam passes, the spacingbetween said parallel internal walls being less than the distance fromthe base of one of said pair of truncated cones to the base of the otherof said pair of truncated cones.

5. A klystron tube having an evacuated envelope, said klystron tubecomprising an electron gun serving to project a beam of electrons, acollector spaced from said gun and adapted to receive said electronbeam, a drift tube interposed between said gun and said collector andthrough which said beam passes, said drift tube including a pluralityofsections forming at least one gap, a cavity resonator surrounding saidgap, said cavity resonator having a first portion internal of saidevacuated envelope and a second external portion, and a dielectricwindow forming a part of the evacuated envelope and serving toelectrically couple the first and second portions of the cavityresonator, said second portion of the cavity resonator including spacedparallel walls forming a box-like enclosure having a given transversedimension, and said first portion including internal surfaces whichdefine a pair of cones coaxial with said drift tube and with theirapexes extending transversely of said box-like enclosure toward oneanother and their bases extending to opposite ends of the dielectricwindow, said given transverse dimension being less than the distancebetween the bases of said pair of truncated cones, and tuning meansmovable toward and away from the dielectric window carried in theexternal portion of the cavity.

6. A klystron tube having an evacuated envelope, said klystron tubecomprising an electron gun serving to project a beam of electrons, acollector spaced from said gun and adapted to receive said electronbeam, a drift tube interposed between said gun and said collector andthrough which said beam passes, said drift tube including a plurality ofsections forming at least one gap, a cavity resonator surrounding saidgap, said cavity resonator having a portion external of the evacuatedenvelope and a portion internal to the evacuated envelope, a ceramiccylinder forming a part of the evacuated envelope and serving toelectrically couple said first and second portions of said cavityresonator, said internal portion including interior wall surfaces whichdefine the surfaces of a pair of truncated cones coaxial with said drifttube and with the apexes of the cones extending towards one another, thebases of said cones being adjacent opposite ends of said ceramiccylinder and having a diameter substantially equal to the internaldiameter of said ceramic cylinder, said external portion of said cavityresonator comprising a pair of spaced parallel walls, said ceramiccylinder having a length greater than the spacing between said parallelwalls.

7. A klystron tube having an evacuated envelope, said klystron tubecomprising an electron gun serving to project a beam of electrons, acollector spaced from said gun and adapted to receive said electronbeam, a drift tube including a plurality of sections forming a pluralityof gaps, cavity resonators surrounding each of said gaps, said cavityresonators having a first portion internal of the evacuated envelope anda second portion external thereto, and dielectric cylinders forming apart of the evacuated envelope surrounding said gaps and coaxial withsaid drift tube, said dielectric cylinders serving to electricallycouple the first and second portions of the cavity resonator, the firstportion of at least one or" said cavity resonators including interiorsurfaces which define a pair of cones having their axes coincident withthe axis of the drift tube and their apexes extending towards oneanother, the bases of each pair of said cones being adjacent oppositeends of one of said dielectric cylinders and having diameterssubstantially equal to the internal diameter of said dielectriccylinder, the second portion of said cavity resonators including a pairof walls defining spaced parallel internal surfaces perpendicular to theaxis of said drift tube, the length of said dielectric cylinders beinggreater than the spacing between said internal surfaces, and tuningmeans movable towards and away from the dielectric window carried in theexternal portion of said cavity.

8. A klystron tube having an evacuated envelope, said klystron tubecomprising an electron gun serving to project a beam of electrons, acollector spaced from said gun and adapted to receive said electronbeam, a drift tube including a plurality of sections forming a pluralityof gaps, cavity resonators surrounding each of said gaps, said cavityresonators having a first portion internal of the evacuated envelope anda second portion external thereto, and dielectric cylinders forming apart of the nators includinga pair of walls defining spaced parallelsurfaces, and the first portion of at least one of said cavityresonators including interior surfaces which define a pair of truncatedcones having their axes coincident with the axis of the drift tube andtheir apexes extending towards one another, the bases of said conesextending outwardly to adjacent opposite ends of the dielectric window,the spacing between the bases of the conical surfaces being greater thanthe spacing between said parallel surfaces. 7 9. A beam tube as claimedin claim 4 wherein a turn ing means movable toward and away from thedielectric window is carried in the external portion of the cavity.

References Cited in the file of this patent UNITED STATES PATENTS2,250,511 Varian et a1. July 29, 1941 2,413,244 North Dec. 24, 19462,415,749 Malter Feb. 11, 1947 2,629,066 Eitcl et al. Feb. 17, 19532,824,289 Murdock Feb. 18, 1958

